Phosphate investment compositions and methods of use thereof

ABSTRACT

Methods for making molding casts by a process typically referred to as “investment casting” using a novel composition is described. The composition is a phosphate bonded material that contains mono-ammonium phosphate (MAP), magnesium oxide and silica filler (quartz, cristobalite, or a mixture thereof). It has been discovered that controlling the amount and ratio of these components has a significant impact on gas permeability (porosity), set time, cast smoothness and cast softness. The most outstanding feature of the new investments is the short mold fabrication and processing time that they allow due to their high gas permeability. The investments can be placed in a burn out furnace while still wet and can be heated rapidly to the firing temperature. The casts can be broken by sandblasting the casts with glass beads. Molds made from the casts have smooth surfaces indicating that the casts are free of visible inclusions and other defects.

RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. patentapplication Ser. No. 09/891,275, filed Jun. 27, 2001, which is acontinuation of U.S. patent application Ser. No. 09/799,016, filed Mar.6, 2001 and abandoned Jun. 27, 2001, which claims the benefit of U.S.Provisional Application Serial No. 60/187,025 filed Mar. 6, 2000.

TECHNICAL FIELD

[0002] The instant invention is directed to methods for making moldingcasts by a process typically referred to as “investment casting” or“lost wax casting.” More specifically, the instant invention is directedto methods of using gas permeable phosphate bonded investmentcompositions that can be burned out rapidly, without fracturing, toproduce strong defect-free molds.

BACKGROUND ART

[0003] “Investment casting” or “lost wax casting” is a process employedin a number of industries to make small parts out of metal, glass,porcelain and other ceramic materials. For example, the process isheavily employed in the dental industry to make dental prostheses and isutilized in the jewelry and gun industries to make small metal parts.

[0004] The first step in investment casting is to create a wax model ofan object to be reproduced. This model is affixed to a wax pin called a“sprue pin” which, in turn, is positioned on a base generally made ofrubber or plastic. A cylindrical ring of metal, plastic, or paper isthen placed on the base so that a cylindrical cavity is created with thewax model positioned in the center of the cavity.

[0005] In the next step, investment powder, which consists of refractoryfillers and a self-hardening refractory binder, is mixed with a liquid,commonly water or a silica sol, to produce a thick slurry. Theinvestment slurry is poured into the cavity formed by the cylinder andbase and over the wax model and sprue pin. The cylindrical cavity iscompletely filled with investment slurry so the model and sprue pin arecompletely covered. The investment slurry is then given time to “set”(harden) and dry. Drying can be accelerated by forced air and othertechniques.

[0006] After the investment has set and dried, the base is removed andthe mold (at this point consisting of the wax model, sprue pin, andoverlying investment) is placed in a “burn out” furnace. The burn outfurnace fires the mold at very high temperatures. The intense heatcauses the wax model and sprue pin to melt and burn—hence the name “burnout.” In addition the heat removes water and other volatile components.Finally, the heat chemically alters the investment material. Obviously,the type of chemical alteration that takes place will depend on the typeof investment material utilized.

[0007] The mold is then removed from the furnace. The end result is amold with a cavity that can be used to reproduce a desired object frommetal, glass, porcelain, and other ceramics. To reproduce an object, amolding material (metal, glass, porcelain, ceramic) is heated untilfluid and packed into the cavity. Then the mold is allowed to cool. Whenthis has been accomplished, the mold is then broken and the casting isremoved and trimmed. In the dental industry, when making prostheses, themold material is removed by sandblasting the mold with glass beads.However, other industries use other methods—such as dropping the mold inwater, creating a heat differential between the outside and insidesurfaces of the mold that cracks the mold open.

[0008] There are many types of investments used in the art. Phosphatebonded investments are one type of investment material. Phosphate bondedinvestments are discussed extensively in Ralph W. Phillips, Skinner'sScience of Dental Materials, 8th ed., pp. 406-410, W. B Sander's Co.,Philadelphia, 1982.

[0009] One of the principle drawbacks to “investment” casting is theamount of time it takes to burn out the mold. This time is due tolimitations on the rate at which the temperature can be raised duringburn out.

[0010] If heat is applied too quickly, a rapid ejection of steam andother volatiles cause the walls of the mold to flake. In addition, ifheating is too rapid, radial cracks will develop in the mold due to theresulting heat differential. In fact, if heating is rapid enough, themold may actually explode.

[0011] In order to avoid the aforementioned problems, the temperature ofthe burn out oven is raised very slowly until the final firingtemperature is reached. For molds made from conventional investmentmaterials, the heating cycle is at least one hour. However, burn out canlast as long as four hours. Moreover, the burn out furnace must becooled between runs, further decreasing throughput.

[0012] A few attempts have been made to create investments that can beheated faster without the formation of flakes or cracks. These attemptsalways involve the use of additives to increase the porosity in thematerial. Porosity increases gas permeability, enabling steam and othervolatiles to more easily escape from the material during the heatingstep without causing damage. However, porosity also decreases thesmoothness of the mold cavity walls which may cause inclusions to formin materials molded therefrom. In addition, the inclusion of additivesincreases the complexity of the composition and, thereby, increases thepossibility of other deleterious effects.

[0013] For example, U.S. Pat. No. 4,604,142 describes the use of starchas an additive to a gypsum-bonded investment to create, among otherthings “good air permeability.” Similarly, U.S. Pat. No. 4,814,011describes the use of starch as an additive to phosphate-bonded andgypsum-bonded investments that contain a wide variety of refractoryfillers to provide “good air permeability.” In both patents, thepermeability of the investment is controlled by the use of additives andnot by a judicious control of the composition of the basic investmentformulation.

[0014] In addition, U.S. Pat. No. 5,373,891 describes the use of gaspermeability improving additives to provide gypsum-bonded investmentsthat can be burned out in short periods of time, thereby, producingshorter production cycle times. Once again, the permeability of theinvestment is controlled by the use of additives and not by control ofthe composition of the basic investment formulation.

SUMMARY OF THE INVENTION

[0015] The instant invention is directed to new phosphate bondedinvestment casting materials. The most outstanding feature of the newinvestments is the short mold fabrication and processing time that theyallow.

[0016] For instance, some investments must be thoroughly dried beforethey can be burned out. In contrast, molds made from the instantinvestments can be placed in a furnace as soon as the investment hasset. Although the molds are quite wet at this point, there are noadverse effects. This eliminates much of the processing time required byother products.

[0017] More importantly, most investments must be heated very slowly toprevent damage from volatile formation and temperature differentials.This means that the burn out oven must be cooled between each run. Theslow heating ramps, and intervening cool down ramps, are very timeconsuming. In contrast, the new investments are sufficiently gaspermeable to enable rapid heating. In fact, heating is virtuallyinstantaneous after setting since the new investments can be placeddirectly into a preheated burn out furnace that is maintained at aconstant elevated temperature. This completely eliminates the need forslow heating ramps and intervening cool down cycles. As a result, theburn out process is greatly accelerated. Conventional investment castingtakes as long as four hours whereas casting the new investments takesless than an hour. The new investments decrease processing time at leastthree fold.

[0018] The new investments are well suited to the production of rush oremergency castings as well as routine production. Although theinvestment materials eliminate much of the time required in investmentcasting, they still produce smooth, defect-fee casts that are free ofhairline fractures and other common deformities. The new investmentsproduce molds that are robust enough to be used in the fabrication ofdental porcelain restorations and soft enough that pressed porcelainobjects may be removed from the molds without undue difficulty bysandblasting with glass beads.

[0019] The inventive phosphate bonded investments do not require theaddition of modifying agents that might add undesirable properties.Instead, the invention utilizes conventional ingredients, namely, silicafiller, magnesium oxide, and mono-ammonium phosphate. The discovery liesin the criticality of the amount and ratio of these ingredients. Only byusing the specified components in the specified amounts can one obtainthe right balance of gas permeability, set time, and other properties.Accordingly, it has been discovered that short setting times, high gaspermeability, and sufficient cast softness and smoothness can only beobtained when a phosphate bonded investment comprises silica filler inthe amount of 72% w/w to 80% w/wt, magnesium oxide in the amount of 7%w/w to 15% w/w and mono-ammonium phosphate in the amount of 10% w/w to17% w/w, based on the total weight of the dry investment powder.

[0020] The particle size distribution of the silica filler is important.It should correspond to the ranges listed in the table shown below. USSeries Screen Screen Opening Microns Cumulative % Retained  70 mesh 2120.0-0.2 100 mesh 150 0.0-0.2 140 mesh 106 0.1-0.8 200 mesh 75 1-5 270mesh 53 10-15 325 mesh 45 15-25

[0021] The most preferred particle size distribution for the silicafiller is shown in the following table. US Series Screen Screen OpeningMicrons Cumulative % Retained 140 mesh 106 0.2 200 mesh 75 2 270 mesh 5312 325 mesh 45 20

DISCLOSURE OF THE INVENTION

[0022] The investment compositions of the instant invention arephosphate bonded compositions. Phosphate bonded investments arediscussed extensively in Ralph W. Phillips, Skinner's Science of DentalMaterials, 8th ed., pp. 406-410, W. B Sander's Co., Philadelphia, 1982.

[0023] The binder in such systems is generally a mixture ofmono-ammonium phosphate (MAP) and magnesium oxide. The reaction of thebinder with water or aqueous silica sol can be described as follows:

NH₄H₂PO₄+MgO+5H₂O----->NH₄MgPO₄.6H₂O

[0024] The ammonium magnesium phosphate salt—water complex(NH₄MgPO₄.6H₂O) forms needle like crystals in a short period of time. Itis this crystal formation that sets (hardens) the composition. Setcompositions, however, are not synonymous with dry compositions. If oneneeds to dry the composition it requires significantly more time thansetting. One of the benefits of the instant invention is that drying isnot required.

[0025] Upon heating, the binder undergoes a number of thermal reactions.The final formulation, after burn out, is a composition comprisingcrystalline magnesium pyrophosphate (Mg₂P₂O₇) and some excess magnesiumoxide (MgO).

[0026] The filler in such systems is a refractory filler. In the instantinvention, the filler is silica. Silica does not undergo any chemicalchange during burnout. The silica employed in the instant inventioncontains cristobalite, quartz, or a mixture of the two. There are noparticular limits on the percentages of quartz and/or cristobalite thatare utilized as these concentrations will vary according to the natureof the material being cast. The particle size distribution of the silicafiller is important. It should correspond to the ranges listed in thetable shown below. US Series Screen Screen Opening Microns Cumulative %Retained  70 mesh 212 0.0-0.2 100 mesh 150 0.0-0.2 140 mesh 106 0.1-0.8200 mesh 75 1-5 270 mesh 53 10-15 325 mesh 45 15-25

[0027] The most preferred particle size distribution for the silicafiller is shown in the following table. US Series Screen Screen OpeningMicrons Cumulative % Retained 140 mesh 106 0.2 200 mesh 75 2 270 mesh 5312 325 mesh 45 20

[0028] The component percentages are critical to this invention. It hasunexpectedly been discovered that very small variations in componentpercentages have enormous affect on the investment's setting ability,gas permeability, and the quality of casts made therefrom. Only by usingthe specified components in the specified amounts can one obtain theright balance of gas permeability, setting time, and other propertiessuch as cast softness and smoothness.

[0029] For the purposes of the instant invention, the amount of MAPshould be from 10% w/w to 17% w/w, based on the weight of the dryinvestment formulation. Ideally, the amount of MAP is 14.9% by weight.

[0030] The amount of magnesium oxide should be from 7% w/w to 15% w/w,based on the weight of the dry investment formulation. Ideally, theamount of magnesium oxide is 11.0%.

[0031] The amount of silica filler makes up 72% w/w to 80% w/w of theinvestment. Ideally, silica filler makes up 74.1% of the investment.

[0032] In the preferred embodiment of this invention, MAP, magnesiumoxide and silica filler are the only materials utilized in theinvestments. This is because the addition of other ingredients can havean adverse affect impact on gas permeability and/or setting time and/orcast softness and smoothness.

[0033] The investment powders described in this patent are easily madeby weighing proper quantities of the ingredients and blending them asdry powders in a mixing jar. The investment powder is then packaged in100 g foil laminate pouches that are heat sealed and stored until use. Atypical investment slurry is made by mixing 100 g of the dry investmentpowder, and a mixture of 20 mL of 30% aqueous colloidal silica solutionand 5 mL of distilled water, in a vacuum-mixing bowl. The mixture isstirred by hand with a spatula until the powder and liquid arethoroughly mixed. The mixture is then mixed under vacuum for 30-40seconds.

[0034] The investments can be used in normal lost wax/investment castingprocedures. However, because of the unique advantages provided by theinvestments, the process can be significantly simplified. Thus, theinvention is also directed to an improved molding process.

[0035] The improved process comprises the following steps:

[0036] (1) creating a wax model of an object to be reproduced;

[0037] (2) attaching the wax model to a “sprue pin” that, in turn, ispositioned on a base; placing a hollow cylindrical ring of metal,plastic or paper on the mold base to create a cylindrical cavity;

[0038] (4) mixing an investment powder with water, aqueous silica sol,or another suitable liquid, to create a slurry, wherein said powdercomprises silica, mono-ammonium phosphate and magnesium oxide;

[0039] (5) filling the cylindrical cavity by pouring the slurry over thewax model and the sprue pin;

[0040] (6) allowing the slurry to set, thereby forming a moldintermediate that comprises the wax model, sprue pin, and overlying setinvestment;

[0041] (7) removing the base and placing the mold intermediate into aburn out furnace that has been preheated to at least 700° C. to make afinished mold;

[0042] (8) filling the mold with metal, glass, porcelain or otherceramic which has been heated until it is fluid; and

[0043] (9)cooling the mold and removing the finished casting.

[0044] Unlike previous formulations, molds made from the new investmentscan be introduced into the furnace as soon as the investment has set.Although the molds are quite wet at this point, there are no adverseeffects. This eliminates some of the processing time required byprevious products.

[0045] In addition, one of the benefits of the invention is that theinvestments set very quickly. A typical investment made in accordancewith the instant invention will set within approximately 5 to 20minutes.

[0046] The inventive investments are sufficiently gas permeable toenable rapid heating. In fact, heating can be virtually instantaneousafter the investment has set or hardened. The new investments can beplaced, while still quite wet, in a preheated burn out furnace that ismaintained at constant high temperature. This eliminates the need forslow heat up ramps. It also eliminates the need to cool down the furnacebetween runs. As a result, the burn out process is greatly accelerated.In a production setting, the burn out of a 100 g mold can be completedin as little as 25 minutes at 927° C. and a 200 g mold can be burned outin 45 minutes at the same temperature.

[0047] Preferably, the mold is placed in a burn out furnace that hasbeen preheated to at least 700° C. The molds generally remain in thefurnace for at least 25 minutes and can be removed from the furnaceshortly thereafter. Under normal conditions, burn out is accomplished inless than one hour.

[0048] The end result is a mold that can be used to reproduce a desiredobject from metal, glass, porcelain, and other ceramics. To reproducethe object, the molding material (metal, glass, porcelain, ceramic) isheated until fluid and packed into the mold. The mold is then allowed tocool. Then the mold removed by grinding, sandblasting, or other meansand the casting is trimmed. In the dental industry, when makingprostheses, the mold material is removed by sandblasting with glassbeads. However, other industries use other methods—such as dropping themold into water to create a heat differential between the inner andouter surfaces that cracks the mold.

[0049] The mold formed from the instant investments do not explodeduring burn out and do not contain hairline fractures, surface flakes orother defects. In addition, the surface finish of pressings made frommolds created with the instant investments is quite smooth and free ofvisible inclusions and other defects. This means that the inner surfacesof the mold cavities are quite smooth which is quite surprising sincemost gas permeable materials generate rough surfaces. Pressing made inmolds formed with the instant investments are relatively soft and themolded material can be easily removed from the cast by sandblasting orother techniques.

[0050] To better illustrate the invention, comparative and inventiveexamples are provided below. These examples are for the purpose ofillustration and not limitation. Many other embodiments of theinvention, that are not specifically set forth in the examples, are bothenvisioned and embraced.

COMPARATIVE EXAMPLE 1

[0051] A first investment was formulated to contain 70.0% silica fillers(58.4% quartz and 11.6% cristobalite), 14.1% MgO and 15.9% mono-ammoniumphosphate. Molds made from this material had a tendency to explode whenheated rapidly. In addition, the molds were hard. Therefore, it wasdifficult to remove porcelain pressings from the molds by sandblasting.

COMPARATIVE EXAMPLE 2

[0052] The investment formulation of comparative example 1 wasreformulated by replacing some of the crystalline silica fillers withdiatomaceous earth to provide gas permeability. However, theseformulations were extremely hard to mix and the approach was abandoned.

INVENTIVE EXAMPLE 1

[0053] The investment formulation of comparative example 1 was thenreformulated to reduce the binder content and increase the fillercontent in an attempt to provide greater gas permeability and make theinvestment softer and easier to remove. This next formulation contained74.1% of silica filler and was quite successful. Wet molds made from itcould be introduced into a burn out furnace maintained at 927° C.immediately after setting without any fear of explosion. However,according to several users, the set time was a bit too slow (13.5 min.).

INVENTIVE EXAMPLES 2-5

[0054] Then, a series of investments (examples 2-5) were formulatedwhich all contained 74.1% silica filler (61.8% quartz and 12.3%cristobalite) and varying quantities of MgO and mono-ammonium phosphate.Example 2 contained 10.1% MgO and 15.% mono-ammonium phosphate. Example3 contained 11.0% MgO and 14.9% mono-ammonium phosphate. Example 4contained 12.6% MgO and 13.3% mono-ammonium phosphate. Example 5contained 14.3% MgO and 11.6% mono-ammonium phosphate.

[0055] When these investment formulations were tried, examples 4 and 5set rapidly. In addition, molds made from these formulations werecovered with a white efflorescence after sitting at room temperature forone to three days. These for formulations were found to be quitesuitable for uses where rapid set times are required and the moldsformed are for immediate use.

[0056] Examples 2 and 3 exhibited slower setting properties. However,these formulations were also determined to be satisfactory for use inthe rapid burnout procedure. Neither of these formulations exhibitedefflorescence.

INVENTIVE EXAMPLE 6

[0057] An additional formulation (example 6), whose composition wasintermediate to those of examples 3 and 4, was prepared in order toobtain a more favorable set time. The formulation was further tested inthe porcelain production laboratory at Den-Mat and adapted as thepreferred composition, primarily because its slightly longer set timeproved to be optimal.

[0058] The following table compares the set times of the formulationsdescribed in inventive examples 1-6: Set Times Example 1 2 3 4 5 6 % wt. 9.3 10.1 11.0 12.6 14.3 11.8 MgO % wt. 16.6 15.8 14.9 13.3 11.6 14.1MAP Set Time 13.5 10.6  8.6  6.3  3.9 — (min.) at ˜27° C. Set Time — —13.5 — — 10.8 (min.) at ˜23° C.

We claim:
 1. A method of molding comprising the following steps: (i)creating a wax model of an object to be reproduced; (ii) attaching thewax model to a “sprue pin” that, in turn, is positioned on a base; (iii)placing a hollow cylindrical ring of metal, plastic, or paper on thebase to form a cylindrical cavity; (iv) mixing an investment powder withwater, aqueous silica sol, or other liquid, to create a slurry, whereinsaid powder comprises silica, mono-ammonium phosphate and magnesiumoxide; (v) filling the cylindrical cavity by pouring the slurry over thewax model and the sprue pin; (vi) allowing the slurry to set, therebyforming a mold intermediate that comprises the wax model, sprue pin, andoverlying set investment; (vii) removing the base and placing the moldintermediate into a burn out furnace that has been preheated to at least700° C. to make a finished mold; (viii) filling the mold with metal,glass porcelain or other ceramic which has been heated until it isfluid; and (ix) cooling the mold and removing the finished casting. 2.The method of claim 1, wherein said slurry sets in approximately 10 to11 minutes.
 3. The method of claim 1, wherein said mold intermediate isstill wet when it is placed in the burnout furnace.
 4. The method ofclaim 1, wherein the mold intermediate remains in the burnout furnacefor at least 25 minutes and less than one hour.
 5. The method of claim1, wherein the investment powder comprises the following components inthe following amounts: (a) mono-ammonium phosphate in an amount rangingfrom 10% w/w to 17% w/w; (b) magnesium oxide in an amount ranging from7% w/w to 15% w/w; and (c) silica filler in an amount ranging from 72%w/w to 80% w/w.
 6. The method of claim 5, wherein the particle sizedistribution of the silica filler is: US Series Screen Screen OpeningMicrons Cumulative % Retained  70 mesh 212 0.0-0.2 100 mesh 150 0.0-0.2140 mesh 106 0.1-0.8 200 mesh 75 1-5 270 mesh 53 10-15 325 mesh 45 15-25


7. The method of claim 5, wherein the particle size distribution of thesilica filler is: US Series Screen Screen Opening Microns Cumulative %Retained 140 mesh 106 0.2 200 mesh 75 2 270 mesh 53 12 325 mesh 45 20


8. The method of claim 1, wherein the investment powder consistsessentially of 74.1% silica fillers, 11.0% magnesium oxide, and 14.9%mono-ammonium phosphate and wherein said silica contains 83.4% w/wquartz and 16.6% w/w cristobalite.
 9. The method of claim 8, wherein theparticle size distribution of the silica filler is: US Series ScreenScreen Opening Microns Cumulative % Retained  70 mesh 212 0.0-0.2 100mesh 150 0.0-0.2 140 mesh 106 0.1-0.8 200 mesh 75 1-5 270 mesh 53 10-15325 mesh 45 15-25


10. The method of claim 8, wherein the particle size distribution of thesilica filler is: US Series Screen Screen Opening Microns Cumulative %Retained 140 mesh 106 0.2 200 mesh 75 2 270 mesh 53 12 325 mesh 45 20